Conventional telecommunications networks for conveying voice and other user information have in general relied upon dedicated telecommunications network infrastructure and transmission protocols. However, with the recent explosive growth in digital data transmission, driven in particular by the use of intranets and the Internet, there has been a move towards the use of more generic infrastructure and transmission protocols in the telecommunications industry. This move is driven primarily by the desire for interoperability between telecommunications networks and other data networks, and secondarily by the cost and performance advantages which general data network systems offer over conventional telecommunications systems.
There exist proposals for the replacement of certain parts of telecommunications networks with packet switched networks and in particular with Internet Protocol (IP) networks. For example, telephone exchanges may be interconnected via IP networks for the purpose of carrying both signalling and user voice and data information.
Subscriber telephone terminals in a Public Switched Telephone Network (PSTN) are generally connected to which provide for duplex (i.e. bidirectional) communication. A so-called “hybrid” located at the local exchange converts the bidirectional voice signals from the two-wire lines into unidirectional signals for transmission over four-wire lines used in the inter-exchange trunk connections. Imperfections in the hybrids may allow leakage of signals back to a speaker's telephone from where the signals originated, giving rise to the perception of an echo.
In conventional networks, the problem of echo is reduced by including an echo cancellation device in a telephone circuit if the propagation delay over the circuit exceeds some predefined period (e.g. 15 msec). As the route taken by a telephone circuit is not always predefined, the first exchange in the circuit identifies the “statically” defined delay for next leg and forwards this to the exchange at the end of that leg. The receiving exchange then appends the delay for the next leg to the already accumulated delay and forwards this to the next exchange and so on. When the accumulated delay exceeds the predefined period, a backward message is sent to the originating exchange asking for an incoming or outgoing echo cancellation device to be included in the circuit.
The above process works because in conventional telephone circuits, which use circuit switched traffic channels, the propagation delay over a circuit leg can be predicted with great accuracy. The proposal to transmit telephone voice data between exchanges using a packet switched network upsets this situation as by its very nature packet switched circuits are unpredictable. Unpredictability arises both because a packet may be transmitted between two end points by one of several different routes and because the network uses only a “best effort” to transmit a packet, i.e. if the network is busy a packet may have to wait or may indeed be lost. The propagation delay over a circuit link provided by a packet switched network cannot therefore be statically defined.